One of the main problems of lithography with EUV radiation or soft X-rays (wavelengths between, e.g., 1 and 20 nm) is the contamination of the optical elements on a molecular level, especially that of the multilayer mirrors used in lithography devices, such as are known, for example, from DE 100 16 008. This surface degradation leads to reflectivity losses and imaging errors. There are basically two prevailing processes. One process consists of oxidation of the surface, during which the surface becomes irreversibly destroyed. The other process consists in the build-up of carbon. This second process is reversible.
The degradation of the surface can be caused, among other things, by ions or electrons striking the surface of the optical element. The ions are produced by photoionization of the residual gas atmosphere or the gases introduced for cleaning purposes by the EUV radiation and the soft X-rays. The electrons usually involve secondary electrons escaping under exposure to the radiation.
The problem of surface degradation by charged particles is further intensified when electrical fields are applied in the region of the optical elements by means of electrodes or grids for monitoring and/or cleaning purposes. A more intense degradation occurs under the resulting bombardment with ions or electrons (depending on the orientation of the electric field).
The problem of surface degradation is furthermore intensified when the optical element is used with pulsed EUV radiation or soft X-rays. During each pulse of radiation, the mirror surface takes on positive charge, since secondary electrons are induced. In particular, when electric fields are applied in the region of the optical elements, the optical elements will be adversely influenced. Furthermore, the steepness of the edges of the EUV pulses, which lies on the order of magnitude of 10 to 100 ns, can lead to electromagnetic effects, which can have a detrimental impact on the overall layout in which the optical element is being used.
It is suspected that the charge carriers in nongrounded optical elements on account of the electrical potential created by their escaping from the surface will fall back onto the surface of the optical element.
From JP 2003124111 A, an optical element with a multilayer coating is known. The secondary electrons arising during the radiation exposure, with an energy of 100 eV, can strike against neighboring optical elements and affect them. In order to intercept these secondary electrons, this known optical element can be grounded, or a potential difference can be imposed between the optical element and the wall of the housing, the wall being connected to the positive pole of a voltage source, and one layer of the multilayer is connected to the negative pole.
However, it has been found that, in addition to the photoelectrons in the range of 100 eV, lower-energy secondary electrons also occur in the region of 10 eV, making up the larger portion of the secondary electrons, and they cannot be caught quickly enough with such an electrode arrangement.
Another problem lies in that, when the multilayer coating is grounded in the traditional way, the escaping secondary electrons cannot be replaced fast enough, which may result in an increasing positive charging of the optical element.